maint: misc changes to dev and ci

[yozachar]

• moves pre-commit to hooks group
• uses isort to add grouping comments, instead of ignoring them
• __init__.py is ignored by isort for documentation
• fix missing -*- coding: utf-8 -*- annotations
• update tox configuration in pyproject.toml
• in GitHub actions, main.yml:
  • changed CI job name to PreflightChecks
  • updated poetry version to 1.4.1
  • runs tox with strategy-matrix
• in pages.yml:
  • updated poetry version to 1.4.1
  • fix typos
• makes flake8 ignore .venv

This PR will be closed in an hour.

[yozachar]

There is a redundant tox run for container's python version when using strategy matrix. It will be dealt with later.